The present invention relates to a technique for performing solder bonding through the use of a temperature hierarchy effective in the module mounting of electronic devices, etc.
In Sn—Pb-base solders, soldering has so far been performed by temperature-hierarchical bonding in which parts are soldered first at temperatures between 330° C. and 350° C. by use of high-temperature solders, such as Pb-rich Pb-5 mass % Sn (hereinafter the indication of “masss %” is omitted and only a numeral is recited) solders (melting point: 314–310° C.) and Pb-10Sn solders (melting point: 302–275° C.), and bonding is then performed with the aid of Sn-37Pb eutectics (183° C.) of a low-temperature solder without melting soldered portions. This temperature-hierarchical bonding is adopted in semiconductor devices in which chips are die bonded, and in semiconductor devices of flip chip bonding, etc. In other words, in semiconductor fabrication processes, it has become important to provide temperature-hierarchical bonding between a solder used within a semiconductor device and another solder for bonding the semiconductor device itself to a substrate.
On the other hand, in some products there have been cases in which bonding at a temperature of not more than 290° C. is requited in consideration of the heat resistance limit of parts. As solders in a composition range for high-temperature soldering suited to this requirement in conventional Sn—Pb-base solders, a Pb-15Sn solder (liquidus temperature: 285° C.) and solders with similar compositions can be conceived. However, when the Sn content becomes higher than this level, low-temperature eutectics (183° C.) precipitate. Furthermore, when the Sn content becomes lower than this level, the liquidus temperature rises, with the result that bonding at a temperature of not more than 290° C. becomes difficult. For this reason, even in a case where a secondary reflow solder for bonding to a printed circuit board is a eutectic Sn—Pb-base solder, it has become impossible to avoid the problem of remelting of bonds of high-temperature solder. When Pb-free solders are used for secondary reflow, bonding is performed at temperatures of 240–250° C., which are about 20–30° C. higher than with eutectic Sn—Pb-base solders, and therefore the bonding at a temperature of not more than 290° C. becomes more difficult.
More specifically, at present there is no high-temperature Pb-free soldering materials that permit temperature-hierarchical bonding at a soldering temperature ranging from 330 to 350° C. or at a temperature level of 290° C.
This situation is described in detail below. At present, Pb-free solders are being used in increasingly many applications in terms of environmental issues. In Pb-free solders for soldering to printed circuit boards, eutectic Sn—Ag-base solders, eutectic Sn—Ag—Cu-base solders and eutectic Sn—Cu-base solders are going mainstream. As a result, the soldering temperature in surface mounting is usually in the range of 240 to 250° C. There is no Pb-free solder for a temperature hierarchy on the higher-temperature side that can be used in combination with these eutectic Pb-free solders. As solders that provide the most suitable combinations, there are available Sn-5Sb solders (240–232° C.). However, when temperature variations on a substrate in a reflow furnace, etc., are considered, there is no highly reliable solder on the lower-temperature side that can perform bonding without melting the Sn-5Sb solders. On the other hand, although an Au-20Sn solder (melting point: 280° C.) is known as a high-temperature solder, its use is limited because it is a hard material and its cost is high. Especially, in bonding an Si chip to a material having a substantially different coefficient of expansion or in bonding a large-size Si chip, this solder is not used because it is hard and might break Si chips.